Paging systems, which come in various levels of sophistication, have certain features in common. Individual subscribers to a paging system have pagers for receiving the paging system broadcasts. Each pager has an individual ID, called the capcode, stored in the pager. All the pagers in an area monitor broadcast messages from a central transmitter. Each message is intended for a particular pager and has that pager's capcode associated with the message. Thus, pagers in an area all monitor a certain frequency for broadcasts from the pager system transmitter looking for its capcode. When the capcode is present, that is when the capcode is broadcast by the transmitter, the message associated with the capcode, which typically follows the capcode, is acted upon by the pager.
The most basic type of pager is called a beeper. The message is simply an instruction to actuate the pager causing it to emit the familiar beep, illuminate an LED, or both. Another type of pager system is used to transmit numeric messages, typically a telephone number the recipient is to call. When the individual pager receives that pager's capcode, the accompanying message is typically stored in a read/write memory in the pager and displayed (then or later) on the pager's numeric display. More recently alphanumeric pagers have been introduced. The messages associated with these pager systems include both number and word messages which are received and stored by the recipient pager for display immediately or later.
One pager, called the PMR 2000 and made by Motorola of Schaumburg, Illinois, is used for both individual messages, those directed to an individual pager, and group messages, those directed to a group of pagers. This type of pager can have one or more capcodes stored in the pager at the time of manufacture or stored in the pager by the local pager company. One capcode is the pager's individual ID while any other would be a group ID since more than on pager would have it.
There are several standard formats for paging broadcast. One format is called POCSAG. With this format any length message is possible. Many pager manufacturers have chosen this format because of its flexibility. Pagers using the POCSAG format "listen" for a preamble broadcast from the pager transmitter for 40 msec out of each second. If the transmitter is not active usually only noise will be "heard" by the pager. As shown in FIG. 2, a transmission is initiated by broadcasting a preamble consisting of a square wave having a minimum of 576 reversals. Since the preamble is at least 1.125 seconds in length, every pager will "hear" the preamble signal during its periodic "listening" period.
Following the preamble a 32-bit synchronization code is transmitted. The synchronization used with the POCSAG format code is the binary coded hexidecimal number 7CD215D8. Following the synchronization code are eight frames, called a batch, containing the transmitted information. Each frame is made up of two 32-bit code-words. The 32-bit code words have slightly different formats based upon whether they contain an address, identifying which of the two million pagers the message is intended for, or a message. FIG. 3 shows that a code-word containing an address has the most significant bit set to 0 while a code-word containing a message has the most significant bit set to 1.
Conventional pagers using the POCSAG format do not monitor each frame. Rather, each individual pager monitors only one of the frames, such as frame 5, to save their batteries. Thus, after the synchronization code is received the individual pager will not monitor the transmission until it is time for frame 5 to be transmitted. Since each frame takes 121.6 milliseconds, the pager in this example would start monitoring the transmitting transmission somewhat before 486.4 milliseconds after the end of the synchronization code. After monitoring its particular frame, the pager once again stops monitoring the broadcast until it is time for the next synchronization code to be broadcast, at which time the cycle starts again. Transmission of the synchronization codes allows the pagers to be synchronized with the broadcast to reduce timing errors. If there is no message to be transmitted in any of the eight frames in a particular batch, an idle code-word (7A89C197) is transmitted in each of the two code-words for that frame. This tells the pagers monitoring that frame that no message is being transmitted for those pagers during that batch.
Under the POCSAG format capcodes are 18 bits long. Therefore, up to 262,144 different capcodes are available. However, since each pager "listens" to one of eight frames, each capcode can be used eight times as an individual ID, once for each frame. With the POCSAG format the total number of capcodes possible is therefore over two million.
One drawback of the presently available pager systems is that they are not flexible. They are intended to provide the same message to one recipient or to a preset group of recipients only. Once the capcodes are set by the manufacturer or local paging company, the are not easily changed or removed from a pager.